/// <summary>Runs the Greedy search algorithm algorithm on a graph.</summary>
/// <param name="start">The node to start at.</param>
/// <param name="neighbors">Step function for all neigbors of a given node.</param>
/// <param name="heuristic">Computes the heuristic value of a given node in a graph.</param>
/// <param name="goal">Predicate for determining if we have reached the goal node.</param>
/// <returns>Stepper of the shortest path or null if no path exists.</returns>
public static Stepper <T> Greedy(T start, Neighbors neighbors, Heuristic heuristic, Goal goal)
{
// using a heap (aka priority queue) to store nodes based on their computed heuristic value
Heap <Greedy_Node> fringe = new HeapArray <Greedy_Node>(
// NOTE: I just reversed the order of left and right because smaller values are higher priority
(Greedy_Node left, Greedy_Node right) => { return(Compute <Math> .Compare(right.Priority, left.Priority)); });
// push starting node
Greedy_Node start_node = new Greedy_Node(null, start, default(Math));
fringe.Enqueue(start_node);
// run the algorithm
while (fringe.Count != 0)
{
Greedy_Node current = fringe.Dequeue();
if (goal(current.Value))
{
return(Greedy_BuildPath(current));
}
else
{
neighbors(current.Value,
(T neighbor) =>
{
Greedy_Node newNode = new Greedy_Node(current, neighbor, heuristic(neighbor));
fringe.Enqueue(newNode);
});
}
}
return(null); // goal node was not reached (no path exists)
}
// methods
#region private static Stepper<T> Greedy_BuildPath(Greedy_Node node)
/// <summary>Builds the path from resulting from the A* algorithm.</summary>
/// <param name="node">The resulting final node fromt he A* algorithm.</param>
/// <returns>A stepper function of the computed path frmo the A* algorithm.</returns>
private static Stepper <T> Greedy_BuildPath(Greedy_Node node)
{
Greedy_PathNode end;
Greedy_PathNode start = Greedy_BuildPath(node, out end);
return((Step <T> step) =>
{
Greedy_PathNode current = start;
while (current != null)
{
step(current.Value);
current = current.Next;
}
});
}
private static Greedy_PathNode Greedy_BuildPath(Greedy_Node currentNode, out Greedy_PathNode currentPathNode)
{
if (currentNode.Previous == null)
{
Greedy_PathNode start = new Greedy_PathNode(currentNode.Value);
currentPathNode = start;
return(start);
}
else
{
Greedy_PathNode previous;
Greedy_PathNode start = Greedy_BuildPath(currentNode.Previous, out previous);
currentPathNode = new Greedy_PathNode(currentNode.Value);
previous.Next = currentPathNode;
return(start);
}
}
public Greedy_Node(Greedy_Node previous, T value, Math priority)
{
this._previous = previous;
this._value = value;
this._priority = priority;
}
